One major goal of plasma spraying and plasma treatment of materials may be generating of stable plasmas having a capability to control within a relatively wide range the heat and momentum transfer to feedstock, thus providing desirable parameters (temperature, velocity, etc.) of feedstock to form a deposition with required properties. Additional goals my include control of substrate temperature as well as other conditions of a deposit formation.
Heat transfer from plasma to feedstock may be characterized by Heat Transfer Potential (HTP) which is the major parameter determining plasma ability to heat particles and substrate:HTP(T)=∫T0Tλ(T)dT where λ is plasma thermal conductivity; T is plasma temperature. HTP may have a correlation with plasma specific power (SP), plasma enthalpy H and plasma temperature. Plasma specific power SP, related plasma enthalpy H and thermal efficiency η may be determined as follows:SP=U*I/Gp; H=SP*η; η=1−Lw/(U*I)where Lw is power losses into cooling media (water); U is plasma torch voltage; I is plasma current; and Gp is plasma gas total flow rate. Specific power SP may be directly measured to characterize plasma conditions and further calculations of plasma HTP, H and temperature. So, SP will be used below for plasma characterization. It may be noted that sometimes power source or control system voltage readings are used as the torch voltage for the calculations. In this case calculated SP and H may be slightly above the real values due to a voltage drop in power cables connecting a power source with a torch.
FIGS. 1 and 2 illustrate the correlations between plasma HTP, H and temperature for argon and N2 based plasmas with input data taken from the literature, such as Thermal Plasmas: Fundamentals and Applications, Volume 1, Boulos, Facuhais, Pfender, Plunum Press, New York (1994)(“Thermal Plasmas”). Generally, increase of plasma specific power and related enthalpy results in increasing of plasma temperature and HTP. However, for N2 based plasmas, plasma temperature corresponding to a particular HTP and related enthalpy may be significantly lower than plasma temperatures corresponding to argon-based plasmas under the same conditions. This is due to the energy needed for the molecule dissociation. For instance, at HTP≈8000 W/m argon based plasmas may have temperatures exceeding 9000° K while nitrogen based plasmas temperature may be below 7000° K (see FIGS. 1 and 2). CO2 and Air plasmas may have even slightly higher HTP in comparison with N2 plasma at the same plasma temperature.
For the major part of plasma treatment of materials feedstock injection into a plasma jet generally takes place downstream of anode arc root attachment and, very often, even downstream of the plasma torch nozzle exit. It may be noted that HTP may decrease significantly downstream of the anode arc root attachment plasma jet due to plasma radiation as well as plasma mixing/interaction with ambient air downstream of the nozzle exit. Decreasing of HTP may result in decreasing of feedstock active dwell time td when HTP value is sufficient for effective heat transfer from plasma to the feedstock. Intensity of plasma interaction with ambient air may be controlled by plasma velocity including velocity distribution as well as by radial and tangential components of plasma velocity. Plasma radiation heat losses Qr mainly depend on plasma temperature and may be estimated using a formula:Qr˜ε(T,P)σSBT4 where σSB is Stefan-Boltzmann's constant; ε(T,P) is “degree of greyness” and ε=1 corresponds to the “absolute blackbody” radiation. It may be noted that for the typical plasma-spray parameters ε is much less than 1 and rapidly grows with the temperature and pressure. Thus, the actual temperature dependence of the radiation flux could be significantly stronger than T4.
Estimates may show that plasma jet radiates so intensively above plasma temperature T≈9000-10000° C. that all additional energy heating plasma above this temperature may be lost within 2-3 cm downstream to nozzle exit and plasma temperature may become there below T≈9000-10000° C. with related decreasing of HTP and enthalpy. Thus, based on FIGS. 1 and 2 as well as on Table 1 below it may be concluded that the HTP of Ar based plasmas having up to 20 vol. % H2 may not exceed HTP≈7-10 kW/m at distances above 2-3 cm downstream of a nozzle exit. For Ar-50% H2 HTP may achieve 14-16 kW/m. However, plasmas with more than 25-30 vol. % of hydrogen with Argon may have a significant pulsing of plasma parameters that their practical application may be very limited.
N2 based plasmas may provide significantly higher HTP at the same temperatures utilized for Argon. For example, N2—H2 plasma having 20 vol. % of H2 may provide HTP up to 18 kW/m before plasma temperatures may achieve T≈9000-10000° C. and the radiation may dominate in the energy balance thus resulting in extremely fast decrease of HTP. The same may be stated regarding plasmas based on other molecular gases like Air and CO2.
TABLE 1Plasma HTP and Enthalpy corresponding to 9,000-10,000 Kplasma temperature for argon and nitrogen based plasmasPlasma gas (vol. %)Ar +Ar +N2 +N2 +Ar20 H250 H2N220 H250 H2HTP, kW/m2-37-10 14-1612.8-14.815.9-18.317.5-20 5H, kJ/g5-610-11.524-2748-5354.8-60  58-64.5
In addition, it has been observed that in the case of Ar—H2 and N2—H2 plasma jets, it may be seen that length of high temperature/high HTP core part of Ar—H2 plasma jet is significantly shorter than N2—H2 one due to intensive radiation. It may result in significantly shorter feedstock active dwell time td and related lack of heat transfer to the feedstock. Thus, it may be concluded that only molecular gases based plasmas may be beneficial when high SP, H and HTP as well as long active dwell time are needed to achieve desirable properties of a deposit. However, it may be noted that molecular gases based plasmas may cause excessive wear of electrodes, plasma instability, pulsing and drifting. With respect to plasma systems, different approaches may be used to avoid or minimize these disadvantages. For example, different plasma passage configurations have been used to stabilize anode arc root axial position of the plasma apparatus thus minimizing voltage pulsation due to the arc shunting. Reference is made to U.S. Pat. Nos. 4,841,114 and 6,114,649. It may be noted that presently the PLAZJET® system manufactured by Praxair-TAFA and having maximum power of about 200 kW may generate stable high SP molecular gases based plasmas simultaneously providing long life of electrodes. For N2-H2 plasmas maximum reported SP level may be of about 42.5 kJ/g.